High molecular weight hydrocarbon soluble polymers, such as polyisobutylene, polystyrene, and several .alpha.-olefins, have been demonstrated to reduce drag in turbulent flows of hydrocarbon liquids. Generally, the drag reduction effectiveness of these polymers improves with increasing molecular weight; however, the tendency for the polymers to permanently degrade via molecular scission in local extensional flows within pumps or turbulent pipeflows also increases with increasing polymer molecular weight. This invention discloses efficient drag reduction in hydrocarbon liquids resulting from a novel class of interpolymer complexes containing, for example, a styrene/vinyl pyridine (SVP) random copolymer and a randomly sulfonated ethylene propylene diene monomer (S-EPDM) copolymer. These complexes can provide improved drag reduction via enhanced molecular interactions rather than increased molecular weight and, consequently, may be less sensitive to flow degradation.